Avoid heroku server from sleeping

It is perfectly legal to keep your free dyno awake for as long as you want, by any means at your disposal. Just note that your monthly allocation of free dyno hours is limited. If you verified your Heroku account with a credit card, you have 1000 free dyno hours per month, which is more than enough to keep a single free web dyno up "forever" without sleeping at all. A very easy way to do that is simply to configure a New Relic add-on for your app (using the free New Relic plan), which you can easily configure to ping your Heroku app periodically. For additional methods to prevent your Heroku app from idling, see here.

Try Kaffeine it will prevent your app from sleeping. Heroku obliges free apps to sleep 6 hours. Kaffeine gives you the possibility to choose when your app sleeps